1. Field of the Invention
This invention relates to maintaining the planarity of biaxially-oriented heatset polyester films. More specifically, the invention relates to simultaneously heat relaxing such films at a uniform temperature and supporting or buoying them by impingement with gas.
2. Prior Art
The commercial use of polyester films, such as polyethylene terephthalate films, is commonplace. In a typical preparation, a melt of polyester is extruded onto a cooling surface, and then stretched lengthwise and transversely (biaxial orientation). The degree of stretch or orientation is "locked-in" by heatsetting, which subjects the film to high temperatures and restraint in the transverse and length directions. This increases the crystallinity of the polyester. As an indication of increased crystallinity, one practicing heatsetting typically controls the degree of heating by monitoring film density. Crystallinity, in this regard, is generally understood to be linearly related to film density (typically expressed in gms/cc). In the case of polyethylene terephthalate, optimum heatset density is in the range from about 1.385 to about 1.395 gms/cc.
Biaxially-oriented polyester films, although heatset, retain a memory of their unstretched state. Without further treatment they will shrink if later employed at high temperatures. This can be avoided by heat relaxing or preshrinking the film at temperatures below those where heatsetting occurred, and by simultaneously allowing the film to shrink (relax) in at least one of the stretched dimensions.
It is generally recognized that heat relaxation can be enhanced by subjecting a film to minimum lengthwise tension on cushions of gas. The objective, in this regard is to provide for least resistance to shrinkage of the film. This will also avoid damage to the film, as the shrinking of the film in contact with hot rollers can create film surface imperfections. See British Pat. No. 858,154 (published Jan. 11, 1961 and issued to Societe D'Applications Industrielles Plastiques); and U.S. Pat. No. 3,632,726 (issued Jan. 4, 1972 to K. L. Knox et al).
In the photographic industry, flatness, or planarity, is often essential in films which have been subjected to stretching and heat treating as described above. The defects that are associated with lack of planarity are numerous. In particular, the invention as described hereinafter provides for the reduction of so-called "drawline" planarity defects. Drawlines consist generally of parallel ridges and valleys extending in the longitudinal direction of the web and uniformly repeating as one views a transverse cross section of the web. They occur predominantly in biaxially-oriented heatset polyester films as a consequence of heat relaxing in a particular manner. Specifically, when these films are buoyed by gas in an environment of uniform temperature, in a flat or gently curved configuration, as described in the aforementioned British Pat. No. 858,154, while under low tension in the film longitudinal dimension, drawlines appear. In addition, drawlines seem to be more severe at higher heat relaxation temperatures, particularly from 30 to 100 centigrade degrees above the glass transition temperature of the polyester under treatment.
U.S. Pat. No. 3,632,726 describes a method of eliminating planarity defects in polyester films. In particular, this patent discloses heat relaxation of polyester film and simultaneous buoying of the film away from solid surfaces on cushions of air. To remove defects the polyester film is heat relaxed in a nonuniform temperature gradient established between a heat source and heat sink. By impinging the film with gas from the heat sink side of the film, those areas of the film with defects are urged closer to the heat source and so are heated to a higher temperature. This differential heating eventually causes the defects to shrink toward and merge with the remainder of the film. In contrast to the teachings of this patent, the invention described herein reduces planarity defects solely by controlling the film's geometry in a region of heat relaxation that is maintained at uniform temperature.